1. Field of the Invention
The present invention relates to an electric compressor and is suitable for enabling efficient operation of an electric compressor for a vehicular air-conditioner at the time of cold startup of the vehicle.
2. Description of the Related Art
Conventionally, vehicles, houses, etc. have been provided with air-conditioners for suitably adjusting cabin or room temperature. An air-conditioner is provided with a refrigeration cycle comprised of a compressor, a condenser, an expansion valve, and an evaporator. The compressor compresses a gaseous refrigerant to obtain a high temperature, high pressure gas, while the condenser robs heat from the high temperature, high pressure gas to convert it to a low temperature, high pressure liquid. Further, the expansion valve converts the low temperature, high pressure liquid to a low temperature, low pressure liquid, while the evaporator causes the low temperature, low pressure liquid to vaporize so as to cool the surroundings. The compressor is generally an electric compressor which is driven by an electric motor. The rotational speed of the electric motor is determined by an inverter device which is controlled by an electronic control unit.
The electronic control unit uses startup/stopping information of the electric compressor which is input from the outside and operating information on what speed to make the electric motor operate as the basis to compute a control signals for the inverter device. The electronic control unit sends the computed control information to the inverter device to control the speed of the electric motor. Such control of the speed of an electric compressor (electric motor) using an inverter device is disclosed in Japanese Patent Publication (A) No. 2000-318435. Further, an electronic control unit which inputs stopping information of the electric compressor to the inverter device wherein this stopping information is made to bypass an electronic control means inside the inverter device and be transmitted to a drive circuit of the electric motor is described in Japanese Patent No. 4151439.
FIG. 1 shows an example of the general configuration of an electric compressor 100 which operates by control signals (commands) from an electronic control unit 5 which is provided outside the electric compressor 100 (hereinafter referred to as the “external ECU”) 5. The electric compressor 100 is comprised of a compressor 3, an electric motor 2, and an inverter device 1. The compressor 3 compresses a gaseous refrigerant to a high temperature, high pressure gas, while the electric motor 2 drives the compressor 3. Further, the inverter device 1 controls the speed of the electric motor 2. The electric motor 2 is usually a 3-phase AC motor. The inverter device 1 is supplied with power from a power supply unit 4 at the outside. When the electric compressor 100 is mounted in a vehicle, the power of the power supply unit 4 comes from a battery 8. Power from the battery 8 is supplied to the inverter device 1 through switches 6 and 7 which are turned on when a not shown ignition switch is turned on.
Inside the inverter device 1, there are an input filter 10, a switching circuit 20, a drive IC 30, and an output signal control IC 40. The switching circuit 20 converts direct current from the power supply unit 4 to a three-phase alternating current. The drive IC 30 controls the operation of the electric motor 2. The output signal control IC 40 is connected to the drive IC 30 by first signal transmission circuits 21 and permits or cuts off output of the drive signals from the drive IC 30. The input filter 10 has a coil 11 and a capacitor (usually an electrolytic capacitor) 12. The switching circuit 20 is provided with a bridge circuit which is comprised of six thyristors, transistors, or other switching devices. Further, the drive IC 30 has a control unit 31 and a ROM/RAM check unit 33 which checks the ROM/RAM region 33. The output signal control IC 40 has an output permission/cutoff control unit 41 which is provided with six on/off switches 42. The output signal control IC 40 and the switching circuit 20 are connected by second signal transmission circuits 22. Further, the output circuits 24 from the switching circuit 20 to the electric motor 2 are provided with a current sensor 34 which detects the drive current. The detected drive current of the electric motor 2 is input to the control unit 31 of the drive IC 30.
The inverter device 1 configured in the above way receives commands from the external ECU 5, uses the control unit 31 to calculate the drive signals of the switching devices inside of the switching circuit 20, and turns the switching devices on and off to drive the electric motor 2. Further, it feeds back the motor current which was detected by the current sensor 34 to the control unit 31 to detect over current. If a motor current which exceeds a threshold value is flowing, the external ECU 5 immediately stops the drive operation of the electric motor 2 to protect the switching devices to prevent malfunctions.
The coil 11 and the capacitor 12 which form the input filter 10 which is provided at the input part of the switching circuit 20 cause the inflow of ripple current of the power supply which is input from the power supply unit 4 to attenuate to thereby smooth the power. Furthermore, the control unit 31 monitors for malfunctions of the ROM/RAM region 33 at all timings and returns the monitoring results to the external ECU. Further, to prevent runaway operation or malfunction of the drive IC 30 from causing the electric motor 3 to constantly consume power, the external ECU 5 directly inputs to the output signal control IC 40 a signal instructing permission/cutoff of output so as to control the on/off switches 42 and monitors the driven/stopped state of the electric motor 2.
However, in the inverter device 1 configured in this way, the external ECU 5 is depended on to prevent runaway operation due to malfunction of the drive IC 30. There is therefore the problem that the inverter device 1 itself cannot be used to prevent runaway operation due to malfunction of the drive IC 30. Here, the case where an aluminum electrolytic capacitor is used for the capacitor 12 of the input filter 10 will be considered. In this case, if the drive IC 30 erroneously operates and applies drive signals larger than envisioned to the switching devices, an extremely large surge voltage which exceeds the withstand voltage may be applied to the capacitor 12 or switching devices resulting in malfunctions. This is because an aluminum electrolytic capacitor increases in equivalent series resistance value at a low temperature. For this reason, the external ECU 5 performs guard control so that the electric motor 2 is not driven at the time of a low temperature. In this regard, in a vehicular air-conditioner which drives an electric motor 2 for heating, if the electric motor 2 is not driven at the time of a low temperature, there is the problem that heating will not be possible and the passengers will become cold.